use @import in <style></style> section in the head element. The only downside is that IE6 might have so called FOUC (Flash of Un-styled Content) problem, meaning the content will be momentarily displayed without CSS formatting. See more at http://bluerobot.com/web/css/fouc.asp

CSS syntax-related stuff:

Contextual selector:

Class selector:

e.g. p.green{color:green;} <p class="green">green text</p>, note that if multiple classes exist, the last one declared in CSS definition file wins.

Id selector:

p#green{color:green;} <p id="green">green text</p>
Difference between id and class selector is that one id value is unique to one element and id is usually used in javascript as well. So, for those styles unique to an element, use id, for styles that can be shared among different elements, use class selector.

Attribute selector:

select element based on attribute existence or values, e.g. this example add a pdf icon after links to pdf files, use [href|="foo"] to specify link names that start with "foo"
a[href$=".pdf"] {background:transparent url(images/iconpdf.gif) no-repeat scroll right center;
padding-right:18px;
}

Unit measurement:

try to use relative units: em (from the width of a character), ex (from the height of character x), or percentage.